Black Mirror Epsidoe 1

A couple of days ago I was watching a TV show called Black Mirror. Somebody posts a video of an abducted princess on YouTbe and the MI5 try to find the person who uploaded it. They ask YouTube and they give them the guy's IP address, but it turns out he's using something like Tor.
Later, one of the technical girls says the following (quoting from memory):
"We weren't able to track his IP but here's what we did. We tried to reverse the compression algorithm YouTube uses to compress the videos and we were able to find the original file size which is 671.5 MB, then we analyzed data from our ISPs and tried to find who had an uninterrupted upstream traffic of 671.5 MB at the video's upload time. We found our guy"